1. Field of Invention
The present invention relates to voltmeters and in particular, voltmeters for use with high voltage transmission lines.
2. Discussion of Background
Voltmeters for high voltage transmission lines are well known. Electric utility employees must test voltages on transmission lines after storms or in other situations. Obviously, voltmeters testing these lines must have sufficient accuracy to alleviate the inherent safety problems with the high voltage lines. The design simplicity of the voltmeter furthers helps in maintaining safety. Various types of voltmeters have been devised for measuring high voltages, such as U.S. Pat. No. 4,594,546 to Green et al. and U.S. Pat. No. 3,969,671 to Smith.
Due to OSHA regulations, electric utility employees must test the nominal line voltage on a de-energized electric line prior to beginning work on the line. Depending on the type of line to be tested, the voltages can range from 200 volts for a secondary distribution line to 14,000 volts for a primary distribution line while primary transmission lines carry voltages up to 300,000 volts. The wide ranges of voltages encountered by utility employees necessitates the use of several voltmeters capable of handling various voltage ranges. A voltmeter with a high resolution is necessary for measuring secondary and primary distribution lines while the high voltages for primary transmission lines require a voltmeter with relatively low resolution.
Various measuring devices, such as ohmmeters and flow meters, have been devised with logarithmic displays; however, the logarithmic output is a natural consequence for these measurements and not produced by additional thought or equipment. An ohmmeter comprises a fixed internal voltage source with a regulating resistor. When a resistor is connected to the ohmmeter, the deflection of the meter movement is inversely proportional to current flowing through the resistor and regulating resistor due to Ohm's Law; consequently, the high-resistance end of the scale is compressed. Also, the energy required to compress the spring in a flowmeter is expressed by a logarithmic equation thereby causing the meter range to be compressed. Although the scale in ohmmeters and flowmeters are naturally compressed, a voltmeter inherently produces a linear output; however, it is necessary to have a voltmeter with a compressed display for high-voltage lines to allow a single meter to measure the wide range of voltages. Therefore, there is need for voltmeter capable of accurately measuring a wide range of voltages while providing a higher degree of resolution at lower voltages than higher voltages.